(1) Field of the Invention
This invention relates to inhibiting and retarding corrosion of metal parts in industrial aqueous-containing systems. More particularly, this invention relates to using a synergistic mixture of certain water-soluble cations to inhibit corrosion of metal parts, especially corrosion of stainless steel exposed to a chloride-ion containing fluid.
Corrosion and cracking in stainless steel results from the simultaneous action of tensile stress and a chloride-oxygen containing environment. The chloride-bearing electrolytes are recognized as being a source of cracking of austenitic stainless steels; and although there is not complete agreement on the mechanism by which the chloride-ion attacks the steel, it is recognized as being the culprit.
Cracking occurs readily in stainless steel specimens immersed in aqueous-containing fluids having only a few parts per million (ppm) of chloride-ions. The temperature of the chloride-ion containing fluids is not particularly significant in producing such cracking as, in addition to producing cracking at boiling temperatures and above, fluid temperatures as low as 30.degree. C. even in such dilute solutions have been known to produce cracking in stainless steel. Furthermore, such cracking appears to occur in specimens both stressed and unstressed; and there appears to be little difference between the resistance of stainless steel to cracking between the various types of stainless steel.
Efforts have been made to eliminate the chloride-ions from fluid systems with which the stainless steel is contacted to thereby reduce stress cracking. In the nuclear reactor field, particularly in the power generating nuclear reactor plant, conduits such as pipes, tubing, pipe fitting, etc., are generally formed from stainless steels; and in such reactors, it is extremely important to eliminate any cracking in the conduits since repair or replacement is difficult or at best extremely costly from the standpont of down-time when corrosion and cracking occur. It is, therefore, the common practice to subject the fluid systems, usually water, used in reactors to extensive purification processes to eliminate the chloride-ion. Due to the vast quantitites of fluids used in the reactors, such purification processes are extremely expensive and utilize equipment of extremely high cost. Furthermore, even though the processed fluids are subjected to such purification processes, some traces of the chloride-ion generally remain so that stress cracking and corrosion cannot be precluded.
(2) The Prior Art
A major problem in industrial fluid handling systems is the corrosion of metal parts which are in contact with the fluids in the system. Such metal parts are for the most part iron and steel, especially stainless steel, although other metals have been used. The use of stainless steel has gained widespread use because of its natural corrosion resistance in many industrial applications, such as boilers, power plants, desalinization plants, electric utility plants and nuclear reactors
Because stainless steel has such widespread usage, considerable research efforts have been made into reducing or eliminating such corrosion since not only is stainless steel expensive, but the cost of plant operations is considerably increased for shutdown and repair of corroded or cracked stainless steel components. Furthermore, the incidence of failures of metal parts is aggravated by chloride-bearing electrolytes. It has heretofore been suggested in U.S. Pat. No. 3,880,585 to Hess et al. that cracking of stainless steel may be substantially reduced by contacting stainless steel with a water-soluble mercuric ion. Although the process suggested in Hess et al. produces significant reduction of cracking and corrosion of stainless steel, it has been found that when the water-soluble mercuric cation is combined with certain other water-soluble metal cations that a synergism results to more effectively reduce corrosion than either metal cation alone.
It is, therefore, the general object of this invention to provide a method for retarding corrosion and cracking of metal parts in a fluid system.
Another object of this invention is to provide a method for reducing stress cracking of stainless steel in contact with a chloride-ion containing fluid.
Still another object of this invention is to provide a method for retarding or inhibiting corrosion of metal which includes a mixture of two water-soluble metal cations to produce a synergistic result.
Yet another object of this invention is to provide a simple, easy method of inhibiting corrosion of metal by contacting the metal with small quantities of water-soluble cations.
Other objects, features and advantages of this invention will become apparent from the following description.